Block editor: keep inner blocks when splitting a selection merges blocks

[ellatrix]

What?

Fixes #26713. Pasting a block that has inner blocks into text no longer drops the inner blocks: a copied list item keeps its nested list.

Why?

__unstableSplitSelection merges the first and last pasted blocks into the surrounding text (introduced in #54543), but the merge only joins attributes: the merged blocks' inner blocks were discarded. This is the paste-side counterpart of #77245, which fixed the merge machinery dropping children on Delete. It also dropped a split half whose attributes were untouched even when it still held children, losing the target's own nested list when pasting at the end of an item.

How?

The head and tail merges carry over the merged block's inner blocks, and an attribute-unmodified tail is kept when it still has children. Following the rule from #78776: the head never carries the first block's own inner blocks, since they sit after its text — within a ranged selection they are deleted with it, and for a collapsed selection they belong to the trailing half.

When pasting at the end of an item whose children belonged to the trailing half, those children now survive on an empty trailing item. Reattaching them to the preceding item would be a separate reparenting decision.

Testing Instructions

1. Create a list with two items, the second containing a nested list.
2. Select the second item as a block (Escape) and copy it.
3. Place the caret at the start of the first item and paste.
4. The pasted item keeps its nested list; previously the nested list was silently lost.

Covered by regression tests for both the caret and the ranged selection case; they fail on trunk.

Use of AI Tools

Written with Claude Code, reviewed and tested by me.

[github-actions]

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Co-authored-by: ellatrix <ellatrix@git.wordpress.org>
Co-authored-by: Thelmachido <thelmachido@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: noisysocks <noisysocks@git.wordpress.org>

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[github-actions]

Size Change: +19 B (0%)

Total Size: 7.76 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-editor/index.min.js	428 kB	+19 B (0%)

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